In the production of pressedboard--a so-called "mat" of particles is prepared by an appropriate deposition or dispensing station, generally upon a continuously moving surface and is possibly subjected to preliminary pressing and thereafter may be compressed to the desired density.
The particles may be sawdust, wood chips or wood fibers or other cellulosic and even mineral or other organic particles and fibers and may contain natural binders or synthetic binders which are added to the mass and are thermally activated by the pressing process.
Depending upon the degree of pressure and heat applied, the products may range in density from comparatively low values to extremely dense products. The low-density materials are suitable for use as insulating board, for example, while the more dense material can be used for load-supporting and structural purposes, e.g. as walls for buildings, in furniture structures and the like.
The apparatus for producing pressedboard can include continuous mat or layer-forming devices wherein, for example, the comminuted material, with or without binders of the type described, is passed between a pair of platens, via a pair of continuously moving surfaces which entrain the material in the form of a layer between these platens and serve to confine the material while it is between the platens.
The platens can be heated and the pressing operation serves to compact the comminuted mass and to harden any thermally activating binders.
When the system produces a continuous mat or layer, the pressedboard leaves the press in the form of a continuous web which is subdivided e.g. by a transversely movable saw, into individual boards downstream of the press.
The reference to "pressedboard" herein should be understood to also include laminates in which at least a core is formed from comminuted materials, i.e. particulates and fibers.
In German patent document (open application) No. 22 15 618, a system is disclosed whereby the layer of comminuted material is carried on a transport belt over the lower platen, is covered from above by a continuous pressing belt interposed between this layer and the upper platen, while between the belt and the respective platens cageless and chainless steel rods roll, these rods forming roller bearings and being circulated from and to the gaps between the pressing belts and the platens. The rods thus function as extremely long roller bearings which can extend the full width of the layer and the belts.
In the system of this German patent document, the diameters of the rods, the thicknesses of the pressing belts and the width of the gaps between the steel rods in the rod-receiving compartments between the pressing belts and the platens are dimensioned so that the belts are subjected to an elastic deformation in the pressing region. The difficulty, however, is that over the long stretches between the platens, the linear path of the rods within these gaps tend to be distorted, the spacing tends to change and the rods tend to bunch up, contact one another and rub against one another although originally they were effectively spaced apart upon entering the compartments.
Obviously this can be partially prevented by utilizing a chain or cage system for holding the rods apart with a predetermined spacing but this has other disadvantages.
As a result of the tendency of the rods or bars to come into contact with one another, i.e. run together, wear develops which can require expensive maintenance and replacement of the rods. In addition the bunched rods or bars tend to jam together and to block continuous movement.
For these reasons practical application of the system has not developed to any significant extent.
In another system represented by Swiss Pat. No. 327,433, the dimensioning is effected such that elastic deformation does not occur and the pressing belts do not deflect between the rods materially. This results in a press operating with pressures of 20 to 50 kp/cm.sup.2 and the relatively thick belts which must be used at such high pressures to avoid the deflection. Naturally such thick belts must be guided on comparatively large diameter drums at least where these belts are directed toward and away from the path of the layer.
Furthermore, experience has shown that even this does not end the problem unless caging of the rods or bars is provided or the bars and rods are connected together in a chain or the like.
The capital costs are therfore high, maintenance costs also high or prohibitive and there is no guarantee that undesired wear will be eliminated.